This invention concerns semiconductor devices and their production.
While there is a considerable interest in InP/InGaAs devices grown by chemical beam epitaxy (CBE), particularly in the area of selective growth, the GaAs/AlGaAs material system is less well established. With the exception of very encouraging data for heterojunction bipolar transistors (HBT's) and high eletron mobility transistors (HEMTS's) the quality of AlGaAs alloys grown by CBE have generally been inferior to those grown by metalorganic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). The degradation of material quality results from unintentional moieties of group III metal alkyls and also unintentional oxygen incorporation. Much effort has, therefore, gone into the development of new precursors which reduce unintentional impurity incorporation in epitaxial AlGaAs layers.
A direct correlation has been established between oxygen concentrations unintentionally incorporated into AlGaAs grown by CBE and trace quantities of diethylether detected by in-situ modulated beam mass spectrometry (MBMS) in the group III metalorganic precursors. The trace ether is residual from the synthesis of the metal trialkyl MR.sub.3, which involves the alkylation of the metal trihalide by a Grignard reagent RMgX carried out in an ether solvent. Subsequent purification processes are then performed to remove the oxygen containing ether solvent and other impurities from the metalorganic precursor. However, these processes are never entirely successful.
For example, U.S. Pat. No. 4,464,233 describes the formation of dimethylmagnesium by reacting a Grignard reagent with a metal halide using electrolysis, having tetra(n-butyl) ammonium percholate as an ionizable support electrolyte and a solvent such as an aliphatic ether, cyclic aliphatic mono- or poly- ether or a non-cyclic ether. Similarly, U.S. Pat. No. 4,604,473 discloses a method of producing a trialkylgallium compound by reacting a gallium trihalide with a Grignard reagent in the presence of an ether.
Trimethylindium compounds with nitrogen-containing Lewis-bases have also been prepared using Lewis-base solvents, such as diethyl ether (see Journal of the Chemical Society, Dalton Transactions vol. 1. 1998, USA; Foster et al: "synthesis and thermal properties of trimethylindium with nitrogen-containing Lewis bases") and the interaction of, for example, ethers and amines with trimethylalane adducts is discussed in Inorganic Chemistry vol. 7, no. 6., Jun. 3, 1986, USA pages 1047-1051; C. H. Henrickson et al.
Alternative methods for producing metalorganic precursors for use in the deposition of epitaxial layers have been described in, for example, U.S. Pat. No. 4,812,586 and EP0460598.